Chemotherapy and radiotherapy can slow the growth and spread of lung tumours, but in most patients the cancer returns and is also generally more resistant to treatment.

Dr Fiona Blackhall, of The Christie cancer hospital in Manchester, and colleagues note there are no tests available that provide early warning about resistance, but they hope their findings might change that.

The team looked at the number of circulating tumour cells, or CTCs, in blood samples of 101 patients with a type of the disease called non small-cell lung cancer before and after they had undergone one cycle of chemotherapy.

They found lung cancer patients with five or more CTCs per 7.5 millilitres of blood had significantly worse survival rates - on average they survived 4.3 months compared to 8.1 months in the case of patients with a lower level of the CTCs.

The findings suggest that counting CTCs could be a simple way to monitor how well a patient is responding to treatment within a few weeks of starting it, say the researchers.

And being able to detect when CTC numbers are rising could give doctors the option to move patients on to new potentially more effective treatments more promptly.

"We now need to test our findings in more patients but, if our results are confirmed, there is now the potential to tailor treatments to individual patients and find new ways to treat the disease," says Dr Fiona Blackhall, a doctor from The Christie cancer hospital in Manchester who worked on the study.

Broader relevance

An Australian expert in cancer control and prevention, Professor Rodney Scott, welcomes the study.

"It's certainly a step in the right direction," says Scott, Head of Medical Genetics at the University of Newcastle.

He says apart from non small-cell lung cancer, which is one of the more common forms of lung cancer, the findings have relevance to other cancers, including breast, bowel and prostate cancers, which represent the majority of diagnosed malignancies.

Scott says researchers commonly believe that the failure of cancer drugs, and the development of metastatic disease, is caused by CTCs, specifically, circulating stem cell tumour cells.

He says that Blackhall and colleagues measured CTCs in general as a surrogate for circulating stem cell tumour cells.

"These are the ones that you really want to get rid of because these are the ones that can lodge in sites and start propagating in those distant sites," says Scott.

Researchers also hope to use gene sequencing tools to study CTCs before and after treatment to learn more about the process that leads to drug resistance, and ultimately to develop new drugs.

Scott says ideally it would be possible to analyse CTCs before the first line of treatment is even given.

"Because you want to give [the patients] the right treatment first time and not put them through something that is going to be less than optimal," he says. "So that's the holy grail."

Lung cancer kills 1.2 million people a year around the world and is one of the lowest survival rates of any cancer because over two-thirds of patients are diagnosed at a late stage when curative treatment is not possible.

More than 80 per cent of lung cancers are caused by smoking, and less than 15 per cent of people diagnosed with the disease survive longer than five years, according to the World Health Organization.